A group of malignant murine cells have been characterized which require a macrophage-derived factor or methylthio groups for proliferation in vitro. These cells are defective in an enzyme which generates methylthio groups from methylthioadenosine in other cells. The proposed research will study the enzymology of methylthio group generation, the role of macrophages in providing these groups to other cells, the function of these groups in cell division, and the possible antagonism of this function by analogs.